In recent years, an electric power steering system has become widespread. In the electric power steering system, a steering assist force is applied to a steering mechanism by driving an electric motor based upon a steering torque with which a driver steers a steering wheel.
For example, a technique described in PTL 1 is known as such an electric power steering system. In this technique, when abnormality of an output value of a torque sensor is detected, an electric motor is controlled by using an alternative value calculated based upon a previous output value of the torque sensor. Accordingly, a rapid assist variation when abnormality occurs at the torque sensor is alleviated.
However, with recent demands for a redundant system of a steering function, a dual safety structure is desired for an electric power steering system. For example, a technique described in PTL 2 is known as such an electric power steering system. In this technique, a sub microcomputer is provided with an abnormality detecting function and a power steering operation is permitted only when a main microcomputer drives a motor in a drive-permitted region, but the power steering operation is prohibited in a drive-prohibited region. Accordingly, serious abnormality in which a steering direction is different from a generated assist torque value is prevented.